This disclosure relates generally to semiconductor devices, and more particularly, to the formation of silicon-on-insulator (SOI) devices.
An SOI substrate includes a bottom substrate underneath a buried oxide (BOX) layer, with a top layer of a semiconductor material located over the BOX. Active devices, such as field effect transistors (FETs), may be formed in the top semiconductor layer on top of the BOX. The presence of the BOX in an SOI device may produce a relatively fast FET device by reducing the capacitance between the source/drain regions of the FET devices on the top semiconductor layer and the bottom substrate. The channel regions of the FET devices, which are located between the source/drain regions, may be decoupled from the bottom substrate by the BOX, allowing movement of the channel region potential with respect to the bottom substrate. For example, when the channel region potential floats positive, the threshold voltage of the FET drops, thereby increasing the FET device drive current.
A difficulty encountered with SOI devices is determining an appropriate BOX thickness. The capacitance between the source/drain regions and the bottom substrate increases as the thickness of the BOX layer is decreased, which may increase the circuit loading. However, the amount of channel region potential movement increases with increasing thickness of the BOX layer due to the reduced capacitance coupling to the substrate, which may result in floating channel regions. A floating channel region may have the effect of producing a fluctuating FET threshold voltage, and therefore an unpredictable device. UTBB (ultra-thin body and BOX) devices may have a relatively thin BOX layer, preventing floating channel regions in a UTBB SOI device.